Liberal leadership candidate Justin Trudeau is slamming an NDP proposal to allow the breakup of Canada through a simple majority in a referendum, saying the plan can only serve to further divide the two opposition parties.

Speaking in Calgary, Mr. Trudeau was reacting to a new NDP bill that would open the door to Quebec secession with a result of 50 per cent plus one, as long as the question is direct and refers clearly to the goal of separation.

"You cannot be half pregnant on the question of Canadian unity," Mr. Trudeau told reporters. "[NDP Leader Thomas Mulcair's] willingness to equivocate, his willingness to be open to a 50 per cent plus one vote on sovereignty takes us back in a direction that we don't want to go. It's a very careful political calculation by him to appease his strong nationalist base in Quebec."

The third-place Liberal Party is facing pressure to open up to talks about further cooperation with the NDP and the Green Party, in a bid to topple the Conservative government in the next election.

But Mr. Trudeau, a Liberal MP in the Quebec riding of Papineau, said the new constitutional proposal by the NDP shows how far the two parties stand on key issues.

"For me it's absolutely unacceptable and yet another example of where cooperation between the Liberals and the NDP in the coming years is out of the question for me," Mr. Trudeau said.

On Monday, the NDP introduced legislation to allow Quebec to secede with a simple majority of 50 per cent plus one. The party also wants to impose a tougher question in the event of a third referendum in the province, such as: “Should Quebec separate from Canada and become a sovereign country?”

The NDP bill aims to replace the 2000 Clarity Act, which was passed by the previous Liberal government after a tight referendum on Quebec sovereignty five years earlier. While it does not include an exact threshold, the Clarity Act calls for a “clear majority” and sends a signal that Ottawa would require a convincing victory by sovereigntists to launch negotiations on Quebec secession.

By agreeing to a simple “majority of valid votes,” Mr. Mulcair is appealing to Quebec nationalists who turned their back on the separatist Bloc and decided to give a chance to the NDP in the 2011 election. While it is clearly federalist, the NDP is also appealing to Quebeckers who felt that the Clarity Act was paternalistic and aimed to unilaterally change the rules of the Canadian constitutional game.

The proposed NDP legislation follows in the footsteps of the party’s 2005 Sherbrooke declaration, while providing a clear sense of how the NDP would handle a third referendum on Quebec sovereignty if it were in government.

“The side that wins wins,” the NDP Leader told reporters on Monday.

The NDP bill goes further than the Clarity Act by proposing the type of question that would be acceptable to the federal government. It has long been a concern among federalists that the Parti Québécois posed vague or convoluted questions in the 1980 and 1995 referendums. In its proposed legislation, the NDP calls on the federal government to go to the Quebec Court of Appeal if the question does not “clearly describe the constitutional change being sought.”

“That’s the rule that’s being followed by the mother of all parliaments in Westminster,” Mr. Mulcair said, referring to an upcoming referendum on Scotland’s independence from Britain.

The Bloc Québécois forced a debate on the Clarity Act with a private member’s bill to repeal the legislation, which is now in front of the House of Commons. The major parties in the House are all opposing Bill C-457, but the NDP is the only one of them to promise that it would replace the legislation with a new “Unity Act.”

Bloc Leader Daniel Paillé was pleased with the NDP’s decision to agree to repeal the Clarity Act, but he was displeased with the party’s decision to oppose the Bloc legislation and come up with its own bill.

“If they want to replace the Clarity Act, I invite them to abolish the Clarity Act,” he said.

Leading a minority government, Quebec Premier Pauline Marois is unable to launch a third referendum on Quebec sovereignty in the near term. However, the PQ Leader continues to promote sovereignty and is planning to meet with Scottish nationalists on Tuesday.

The Conservative government refused to clarify its acceptable threshold to enter into negotiations on Quebec secession. During a debate on the Bloc legislation, Conservative MP Jacques Gourde said the Harper government will not revisit “the debates of the past.”

Liberal MP Stéphane Dion, who was one of the architects of the Clarity Act in the Chrétien government, said his party will not budge on the principles under which it raised the bar in the event of a third referendum.

“If there is clear support for a secession, there is negotiation. If there is no clear support, there is no negotiation and without negotiations, there would be no secession,” he said.

The NDP bill is sponsored by MP Craig Scott. Given the rules governing private members’ bills, it is unlikely that it will be up for debate in the near future.